Physics Day 6: Hover Disc Intro – Connecting motion and forces

This year when I started balanced forces with the hover disc FBD’s (see here for the general idea), I decided to try connected forces and motion more explicitly.

hover disc

First, I turned on the hover disc and pushed it across the circle and then asked the student to push it back to me faster. I asked students to sketch what they think the velocity graph would look like for this motion. Then they compared with their neighbor, and I chose 3 students to sketch their velocity graphs on the board. I chose the following 3 graphs because they were all a little different.


We talked about the similarities between the graphs (all have positive and negative portion, all cross over the time-axis -well maybe not the blue one). And differences (green and blue have constant velocity portions and the red one doesn’t. The red and green one take time to speed up and slow down. The green one shoes the second speed as greater but the blue and red show the speeds as equal).

Then we used the motion detector to check our predictions.


Students noticed some cool things on this graph – the second portion has a greater speed, and it also takes less time to cross the circle (same distance). This is a nice precursor to thinking about the area under the v-t graph.

Then we turned the hover disc off and I pushed it. Students made graph predictions again, and I selected 3 different ones to sketch on the board.image

Again, talk about similarities and differences and then check with motion detector:


After doing this series of graphs, I asked students to think about why the motion was different when the hover disc was turned on vs. turned off. Ultimately we want them to connect constant velocity with balanced forces and changing velocity with unbalanced forces.

Most students talked about the air from the hover disc reducing friction, but of course there were some students thinking that the hover disc has something that propels it forward when it’s turned on. (like a Roomba!) I asked how we could test this, and they said let’s turn it on and set it down on the floor without pushing it. If it doesn’t move, there’s no force propelling it forward. Any guesses as to what happened when we did this? This thing always moves when you set it down. Some said “well the floor’s not perfectly flat!” but that wouldn’t be very convincing to me if I thought it was self-propelling. I suppose we would try setting it down many many times, always with the same starting orientation, to see if it always moves in the same direction, but we didn’t do that.

So the question is – how do students come to understand that there are zero horizontal forces in the first situation? It is hard to prove this empirically. It usually ends with me just telling them this is true, which feels unsatisfying to me.

I do tell them it took a long time for scientists to sort this out, and in a subsequent class I showed them Galileo’s argument in response to the Aristotelian view. A ball rolled down a ramp will continue to roll at a constant speed forever, unless there is some force that acts on it to speed it up or slow it down.


I didn’t explicitly ask students to come up with counterarguments (as David Hammer did in the physics class he taught, described in his Misconceptions vs. P-Prims paper). But, one student did this spontaneously: “If I’m playing devil’s advocate, I can believe the ball would go on forever, but that doesn’t convince me that there’s no force causing it to do that. Wouldn’t Aristotle say the ball must have some sort of internal force to make it keep moving forever?” We (the other students and I) didn’t have a very satisfying response to that. Do you?


Participation Goals

Like a lot of teachers, a small part of the grade in my class is “participation.” Over the past 3 years I have tried to refine this grade so that it’s less about just “talking more.” I made a rubric that defines “Participating in the Scientific Community” with 5 different categories that are focused on students engaging with each others’ ideas.

Screenshot 2015-09-29 08.55.31

Screenshot 2015-09-29 08.57.26

The first time I used this rubric I made the mistake of using the students’ self assessment scores directly in their grade. As you can imagine, students felt a tension between being honest and reflective and wanting to give themselves a high grade. After that, I changed the grade to be based on students’ progress on a goal they set for themselves based on the rubric. In the beginning of each quarter, students self-assess using the rubric and come up with an example of how they have engaged in each category. Then they set a goal for themselves using the following questions:

  1. What is one participation goal you’d like to work on for the remainder of Quarter 1? (You can base it off of one of the categories on the other side of this sheet, or it can be something else, but it should be related to your participation in class)
  2. We will be checking in one-on-one at the end of the quarter to evaluate your progress on this goal. How will we know if you have made progress? (i.e., what observable evidence should I/you be looking for?)
  3. What are some obstacles or challenges you foresee in working toward that goal? What specifically can YOU do to overcome those challenges?
  4. What can I do to help you make progress toward your goal?

I give them feedback on their goal and hand back the sheet for them to keep in their notebook.

For the first time this year, I am going to have the students keep a goal tracking sheet where they reflect on how they’re progressing each day and include an example from class. At the end of the quarter, I will meet with each student individually (for 4-5 minutes) to talk about whether they feel like they’ve met their goal or not. Then their participation grade is based on that conversation.

The most common goals are some variation of “I need to share more” and “I need to listen more.” For the ones who say they need to share more, I’ve noticed an interesting split regarding what they think they need to do to meet this goal.

One group of students say they need to understand the material more before coming to class, which will allow them to participate more:

Student A:

“I’d like to participate more in whole-class discussions. Not understanding the material would lead to not being able to participate. I can make sure I know all the material and try to talk first.”

Student B:

“A goal for myself would be to be more open to sharing more ideas… Some obstacles would be not being confident enough. I will prepare for class more by making sure I really understand the concepts so that I will have some confidence and know what I’m doing walking into class. If I don’t understand a concept, I can set up a meeting with you or ask my classmates.

Another group of students say they need to share their ideas even when they’re uncertain because that’s what leads to learning:

Student C:

“I would like to work on having the confidence to share ideas/answers even when I’m not sure if they are correct. Basically, I don’t want to be too worried about being wrong, because I know that making mistakes is part of learning.”

Student D:

“I want work on sharing without being called on. I want to gain confidence in how I share with the class. I just dislike making mistakes publicly so I have to trust myself more and be okay with making mistakes because that’s the best way to learn”

The following questions come to mind when reading these responses:

What do students think is the purpose of the class discussion? Students A and B seem to think it is to display knowledge, and Students C and D seem to think it is to gain knowledge.

How do students thinking learning happens? Students A and B might think learning happens when reading information or hearing the material explained by a teacher or classmate. Students C and D might think learning happens through talking about your ideas.

One challenge I’m having is how to respond to these ideas. Part of me wants to tell Students A and B that you should share your ideas especially when you are uncertain, because learning happens when you talk through your ideas with others. (and in fact, I wrote that feedback on a lot of their goals sheets!) But why would me telling them something make them change their ideas? That’s awfully inconsistent of me! I know that teaching physics concepts doesn’t work that way, why would teaching about learning be any different?

So that’s the shift that I need to make – that part of my job is to teach about learning science, and that doesn’t mean just telling them how people learn science. Students have epistemological expectations coming into class, and telling them something else won’t change their minds. I have to set up experiences where they can use their productive epistemological resources and help them develop their understanding of knowledge and learning over time.

I will end with one more student’s participation goal. What a great idea for structuring whiteboard discussions!

“I would like to continue working on building off of my peers’ ideas to deepen understanding and gain multiple perspectives on one situation… I should be able to look at a graph [on my group’s whiteboard] and identify the role of my personal thinking that went into producing the graph as well as point out what parts of the graph reflect what my peers thought. It’s important to emphasize the process by which table groups reached agreement on a solution”

Physics day 5: Airplane bowling

I decided to have students discuss this question before starting forces this year:

Screenshot 2015-09-26 20.32.24

I chose this question because I knew students would have lots of good ideas about it, and it’s complex enough that we wouldn’t fully figure it out in one discussion. Plus, I knew we could revisit it a few times this semester and refine our explanations.

Problem was – pretty much all students agreed that the answer was A, so it felt a little artificial to debate. I asked students to think of reasons someone might say B or C, and they came up with good explanations, but it still felt forced.

imageWhat I realized throughout the discussion, though, was that students did disagree about the specific path of the ball, even if they all thought the answer was A.

One disagreement had to do with the shape of the path – curved vs. straight. This I wouldn’t have noticed if these 2 groups hadn’t spontaneously drawn the path when they wrote their explanations:



Another question that came up was whether the ball loses any forward motion to gravity – in other words, does gravity “redirect” the forward momentum of the ball to be more straight down?

After the in-class discussion, I asked students to do the following for homework.

  1. Today we heard lots of ideas in class. Choose one idea that you heard that you disagreed with and describe it here in as much detail as possible. Explain that person’s reasoning. (note: you’re just describing the idea, not saying why you disagree with it)
  1. Why do you disagree with that idea? Why does it not make sense to you?
  1. What is your current thinking on the answer to the question on the other side of this sheet? Why? (If you changed your mind, explain why. If you think pretty much the same thing, explain any new insights you have into the situation)

Note that I made this homework before class, and I expected more students would  disagree about the answer (A, B, or C). But since so many chose A initially, and still thought A at the end, a lot of them said they had no new insights. This was disappointing to me, because I know there were things that came up that they hadn’t thought of – stuff about the path shape, whether the ball will “keep up” with the plane or not – but not many of them mentioned that on the homework.

Next year I want to reword the original question so more of their thinking is made visible, which will provide more fodder for discussion and reflection. I’m thinking about providing a blank film strip (do students know what these are??) and asking them to sketch the airplane + bowling ball from when it’s released to when it hits the ground.

film strip

One question I still have though is – whether to tell them to ignore air resistance or not. It seems weird to say that before we’ve talked about forces at all in class. But if there’s air resistance, it would be hard to tell if their ball is falling behind the plane because of air resistance or because gravity “redirects” the forward motion. The only way to sort those two out is to remove air resistance – maybe I could get the students to see this. I hate saying “we’re going to ignore air resistance because physicists like to start with simple models” and would love to move toward the students deciding we should ignore air resistance for a specific reason (like distinguishing between two different explanations)

Engineering Design Journals

Last year, I had students keep a digital design journal (a Google Doc, one per group). I wanted them to record their progression of ideas and questions as they went through a design challenge. I asked them to include brainstorming, sketches, explanation of any decisions they made, test results, challenges, questions they’re trying to answer, etc.

Excerpt from last year's design journals
Excerpt from one of last year’s design journals

I liked that students could add photos easily to the Google Doc, and that multiple group  members could work on it at the same time. But there were also some downsides. Students often didn’t finish it in class, so they would work on it outside of class. This was a problem because I really wanted it to be keeping track of their messy ideas in the moment. Instead, it was more like a post-hoc report of what happened in class.

Excerpt from last year's design journals

I also think the format made them feel like it should be more like a finished product – like an English paper – since you can edit things so easily in the Google Doc. It  also was hard to include all of their sketches – they had to decide that a sketch was important enough to take a picture of it and upload it (or some of them used SketchUp, which is nice but again, too formal)

Excerpt from last year's design journals

Excerpts from last year’s design journals

So this year I am trying physical design journals that they turn in at the end of class. So far I’m really liking it. They can’t work on it outside of class. They can sketch easily and pass the notebook around while they’re talking and showing each other their ideas. (A Google Doc on a laptop doesn’t really facilitate this easily, even if multiple people have the doc open). The thing I like the most about the physical journals is that they can be messy and annotate their diagrams more easily. Last year, they would include a hand-drawn diagram and talk about it in the text below, but I would have no idea exactly what part of the diagram they were referring to.


I have a vision for what I want these to look like, but am still working on articulating this vision. Instead of creating a rubric or checklist for the journals at the beginning of the project, instead I am using the in-process journals to help me determine and refine those guidelines. Each class, I show some examples of their journals each day and highlight what I like about them and what can be improved.

Design journals after first day of the cantilever project – initial brainstorming

After showing examples of their first day journals, I said I liked the inclusion of lots of ideas, but I need to be able to follow the process of their thinking, so they should also include written description and point out specific things on their sketches that they’re describing. I also want them in include questions they have – like if they’re trying to decide between 2 alternatives, explain that.

This group is thinking about whether glue or balsa wood is stronger and how that will affect their design (see bottom of page)


I asked them to also include any results from testing, and this group decided to add a picture of where their cantilever broke when they tested it and explained how they will reinforce their next design.


This group does a nice job of annotating their sketches and explaining the support they will add next time and why:


Overall, I like how the quality of the journals has improved over the first 3 days of the project, but so far it feels a bit like me making arbitrary guidelines that they try to follow.  I want to talk more about the purpose of these journals. When/why do engineers in the real world use them? Do they go back and refer to previous days’ work? With a clear purpose, it would be easier for students to understand what to include and why.

I also want to shift toward having the students read each other’s journals and ask each other questions and give feedback. I think it would help them see when they they’re not being clear with their ideas if they heard from another student “I have no idea what you’re talking about here!” I’ll write another post after I try this.

Physics Day 4: Mistake Game Mistakes

I tried doing Kelly’s Mistake Game when whiteboarding problems today, but I made the mistake of not re-reading Kelly’s post before trying it. (I actually decided to do it on the fly when I realized that we had extra time in class)

Overall, students thought it was fun, but here are some mistakes I made:

  1. I had all groups whiteboard the same (multipart) problem. This meant that several groups chose the same mistake, and they didn’t have to pay much attention to the presenting group to catch the mistake. Some groups changed their mistake when they saw another group already used it, so they changed it to something trivial
  2. We didn’t spend enough time discussing what “good mistakes” are
  3. My multipart problem was too long so there was a lot of text on each whiteboard – very hard to look at and understand.
  4. I tried a second round where 3 groups got problem A and 2 groups got problem B (to try to fix my mistake #1). These were brand new problems (the class hadn’t been working on them in the packet), so it was hard for “problem A” groups to follow the “problem B” groups’ presentations.

In the second round, I (secretly) told some groups to make a mistake and others to not make a mistake. (I told the class I was doing this), which made it a little more interesting. Next time, I will have each group whiteboard a different problem, and keep them short enough (but still conceptually complicated) that there’s not so much to write on the board.

I’ll also try using problems that everyone has worked at in their packet, although I’m curious about the benefits and limitations of doing this vs. having each group present a “brand new” problem. Anyone tried it both ways and have an opinion on this?

Engineering Day 3: Trade Offs

After designing an improved paper clip last class, students read a chapter from Henry Petroski’s Invention by Design (the text for the course). The chapter was on the history of the paperclip and included lots of patents for new, improved paper clips. Students loved seeing that the patents had many of the exact same ideas they came up with.

One of the themes for the class is considering tensions or trade-offs in any design task. During our projects, students often ask for more time, more materials, and less constraints, and this year I want to be more thoughtful about how we talk about why there must be constraints, and those constraints depend on the context.

I had students draw a diagram that represents all of the tensions present in designing a paperclip, drawing on their experience last class as well as using the information in the chapter. I liked the diagrams they came up with – and they included a lot of tensions that are present in any design project, so I want to revisit these throughout the year.image

Physics Day 3: Position and velocity graphs

Today when position graphs, one student asked how the graph would change if the origin was different. He thought the position graphs would no longer be linear. I had students talk in groups about what the graph would look like if the origin was between A and B’s starting positions. I had a student come draw her new graph on the board. Instead, she just drew shifted axes on the same graph – smart! image

(And after further probing, I learned that the student who had the question was actually thinking about 2-dimensional space, and what if the origin was not on path that A and B were traveling. good question!)

Later, students were concerned about making a velocity graph from a position graph that starts at time t=0. Some students really wanted to use closed circles or open circles for the endpoints of the lines. They thought if the position graph endpoint is a closed circle, than the velocity graph would have to start with an open circle, since you can’t actually find the slope of the position graph at t=0. Idea on how to respond to this other than saying there’s things math teachers care about that physicists don’t? Maybe that the slope immediately after t=0 is essentially the same as the slope at t=0?


Later in class, I was showing how you could solve the position equation for velocity, and you got back to the slope of the position graph. There was an audible gasp from multiple students!


Physics Day 1 & 2: Constant Velocity Buggies

The past 2 years, I started physics with the canonical buggy activity – each group gets a buggy and a starting point and direction and models the motion of the buggy with graphs, equations, and words. Then students come together in a board meeting and look at what was similar and different between their whiteboards to build the key components of the CV model (similar to how Kelly O’Shea describes it here)

Some things I’ve liked about this – students are making measurements, moving around the room, sitting on the floor, and figuring out how to collect data on this own – it feels like controlled chaos (which I like). There’s also not too much guidance from me during the discussion – they can easily pick out the key parts of CV motion.


Two things I haven’t liked about this:

  1. it takes a long time for students to make the measurements by hand, and
  2. there’s no obvious purpose to modeling the motion of this buggy. “Because physicists like to represent and predict motion.” Not very satisfying.

To address concern #2, this year I tried starting the CV unit by asking students to predict the time and location of a buggy collision. (Many people use this activity later in the unit). I told gave each group a fast and slow car and told them the initial separation and asked them to predict the time and location of the collision. I didn’t give much other instruction other than saying they can take measurements of the cars separately but they can’t collide the buggies during their data collection.

Some interesting things came up when students were making their predictions. Some wrote equations for the distance traveled, thinking about rates from math class.

Screenshot 2015-09-05 10.26.43

Some thought about the ratio of the speeds being equal to the ratio of the distance traveled.Screenshot 2015-09-05 10.23.24

Some groups spontaneously thought about graphing the position vs. time without any instruction to do so.

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This group made found the speed of each buggy, and then used the speed to make marks on the floor to show where the buggy is each second, and saw where the collision would be. They got stuck, though, when they couldn’t figure out exactly when the collision would be between 7 and 8 seconds (when the marks crossed each other).


One downside to doing it this way is that all groups assumed each buggy had a constant speed so they only made one speed measurement for each buggy before calculating the collision location (even those who were thinking about graphs). Some groups’ predictions were very close, and some weren’t, and we didn’t have enough evidence to know if the predictions were off because of bad data collection technique or because the car isn’t moving at a constant speed.

But watching the cars, they so obviously look like they’re moving at a constant speed – so again, why is confirming that interesting at all? “Well, later we’ll get into more complex types of motion, but we’ll be able to use the same tools we’re developing here” Not very satisfying.

Students loved watching the collision tests, though, so that’s something at least. When the buggies did actually hit, there was lots of cheering and clapping.

2015.09.05.buggy cheering blurred

This doesn’t address my first concern – that we have faster ways of making these measurements than crawling around on the floor with tape and a stopwatch. Although, I think there’s something nice about asking the students about how they could improve their measurements (after we watch the collision videos) and they say “use the video!” and I say “great, let’s do that!” We draw points on the board every frame.

2015.09.05.lines on board

Then I ask how we could improve the analysis? “What if there was a program that could plot those points? and I say “great, let’s do that!” We use Logger Pro to see the x-t and v-t graphs of motion and talk about what those mean.

Screenshot 2015-09-05 10.06.10

Then I say, well it’s still hard to make perfect dots on the video – human error and all – and what if we didn’t have a video? what if we wanted to make measurements in real time? They usually come up with sophisticated ideas for automated data collection, or just say something like “lasers!” and I say “great, let’s do that!” and introduce the motion detectors (not quite lasers, but close enough). So maybe starting by crawling around on the floor with tape and a stop watch isn’t such a bad beginning to this progression. Students get a physical experience to connect to motion detectors which might otherwise feel like magic.

There’s also another issue with the way I did the collisions – they all started at the same distance away from each other and moved toward each other. Gets them thinking more about distance than position, as Frank Noschese explains nicely in this post from last year. If I do collisions again next year, I will definitely vary the direction and starting points.

In general, I’m still not that excited about starting the year with kinematics at all, and I’m moving toward doing forces first sometime soon. Each year, I get closer to it – this year I did 4 days of Constant Velocity motion, finished with a quiz, and am jumping into balanced forces this week. I think it would be much more satisfying to start the year talking about “how” and “why” questions, and then get back to kinematics when it is useful to us. Have you tried forces first? Are you thinking about it? I would love to hear your experiences/ideas.

Engineering Day 2: Improving the paper clip

Today I asked students to think about all of the problems with the current design of the paper clip.


After generating a list of problems, each group chose 1 problem and designed a new paper clip that would address that problem. After 15 minutes of designing, groups shared their new paper clips to the class.


After each group shared, I let the rest of the class ask questions. These are all seniors and are very comfortable with each other, so they immediately started challenging others’ designs.


“Won’t the rubber ends make it hard to take the paper clip off the paper?”


“Won’t the coils stretch out?”


“Doesn’t that make the paper clip like twice as heavy but it doesn’t hold that much more paper?”


“Didn’t you just reinvent the C-clamp?”

I was glad that they were critical, but it felt a little too much like Shark Tank. This year i want to emphasize more collaboration between groups – last year students were very competitive, which sometimes felt uncomfortable (to me and to some students who talked to me about it). Some competition can be good, but not if it stifles creativity and risk taking.

How do you promote a positive, collaborative atmosphere, especially during engineering design tasks?

Engineering Day 1: Progressive Design Challenge

Day 1 of my engineering class, we start with what I call a “progressive design challenge.”

Students start with 10 straws and 10 paper clips each, and have 5 minutes to make the tallest freestanding structure.


After the first round, I pair students up and they pool their resources (both material and intellectual) and try again.


After the pairs, I had them reflect on process – how did they make decisions about what to do? What features from Round 1 made it into Round 2? What features didn’t? Why?

Then we combined 3 pairs to make groups of 6, pooled resources, and tried again.


At the end of Round 3, I asked them reflect on all 3 rounds and create a diagram that represents their overall process. Instructions were vague, so one group reflected more on their process, specifically how their ideas evolved:

Screenshot 2015-08-24 15.25.28

While the other 2 groups focused more on the evolution of the structure itself. This group explained which features made it into subsequent rounds and why.

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